The present invention relates generally to an improved technique for generating a super set pulse amplitude modulated (PAM) constellation for a computer modem. In particular, the present invention is directed to accommodating Robbed-Bit Signaling (RBS), programmed attenuation (PAD), and Inter-Modulation Distortion (IMD) during a Digital Impairment Sequence (DIL) conducted with V.90 modems or the like.
The V.90 modem is also known as the 56K modem, which, due to power limitations imposed by the FCC is presently limited to 53 Kbits/second transmission rate. FIG. 4 is a simplified block diagram illustrating how a V.90 modem 530 may be connected to a server 510 through a codec 520. V.90 modem 530 is an analog modem communicating with a telephone company (telco) codec (coder/decoder) 520 through a local dial-up line or local loop 540.
As may be appreciated by one of ordinary skill in the art, such a local loop 540 may contain analog impairments characterized as xe2x80x9cloop lossxe2x80x9d. V.90 modem 530 may be provided with an equalizer to offset some of this loop loss. Data transmitted from V.90 modem 530 to codec 520 may be in one of a number of formats depending upon the type of codec.
For example, a so-called xcexc-law codec may receive data in a 13-bit format (as illustrated in FIG. 4). A so-called A-law codec may receive data in a 12-bit format. Other, so-called xe2x80x9cnon-conformingxe2x80x9d codecs may receive data in yet other formats. Regardless of which format the data is received in, codec 520 converts data received from V.90 modem 530 into digital form (typically 8 bits) for transmission over a telco digital trunk line 550 to server 510.
Digital trunk line 550 may itself contain so-called xe2x80x9cdigitalxe2x80x9d impairments, including robbed bit signaling (RBS), digital pad (PAD) and inter-modulation distortion (IMD). Most of these digital impairments are due to design considerations implemented in the telco digital network when it was largely used as a voice-only network. However, such impairments present problems in transmitting digital data over such a network.
The V.90 standard adds to and inherited advantageous features of pre-V.90 56K modems (e.g., X2 modem and Kflex modem). One important feature of the V.90 modem is performance optimization. Using Digital Impairment Learning sequence (DIL) data, the client modem is capable of generating optimal constellations to achieve best throughput for given conditions.
V.90 constellations rely closely on DIL data points. Thus, accuracy of DIL data is one key for accurate constellation generation. One big problem of the prior art is how to obtain highly accurate and optimal DIL data points. V.90 modem 530 must receive a training signal (the Digital Impairment Learning signal, or DIL) and be able to distinguish analog impairments from digital impairments (xe2x80x9cde-noisingxe2x80x9d data) in order to properly characterize the data channel and generate accurate and optimal data constellations.
In the present invention, several techniques are proposed to is de-noise DIL data and to achieve accurate DIL data, including a linear-to-Ucode conversion algorithm, PAD/RBS pattern detection, DIL de-noising processing, and a xc2xd-RBS de-noising process.
By detecting the non-RBS pattern (e.g., no RBS is present), the DIL data points of the non-RBS slots are averaged to increase accuracy of the received DIL data. Similarly, using averages of DIL data with the same RBS-pattern slots, more accurate DIL data points are obtained. Accurate DIL points provide a key for optimal data throughput performances of the modem.
Received linear DIL data points are equalizer outputs and may be rough and noisy. By the linear-to-Ucode conversion, the DIL rough data may be converted to Ucode indices. The Ucode indicies may be matched to the closest G.711 (xcexc-law or a-law) values. This process serves to de-noise the DIL data.
The xc2xd-RBS slot refers to D4 channel bank CODECs specified in ATandT Technical Reference, PUB 43801, November 1982, that output close to mid-values of its normal outputs in the RBS slots. In here the codec transfer characteristic changes to span the entire dynamic range using 7 bits during RBS slot. Present invention matches for the ideal values specified for this type CODEC in the xc2xd-RBS slot. This is noted as xc2xd-RBS de-noising.
The present invention may also detect and eliminate DIL data points which are too noisy and/or non-monotonic. In addition, an upper limit may be set for constellation points to avoid saturation of the receiver, by applying PAD and IMD correction. Ideal DIL data points may be added for typical 0 dB, 3 dB and 6 dB PAD to help create optimal constellation tables and thus optimal modem connections. If PAD-detection has failed, the PAD may be set to 0 dB and the constellation based on originally received DIL data points.